Defibrillation pacing circuitry

ABSTRACT

Electrical circuit componentry is switchable into a defibrillator circuit to deliver a constant pacing current to a patient. The circuitry may include a constant current source inserted in a leg of the defibrillator circuit or a resistor of selected value inserted between a high voltage source and the high side of a defibrillator circuit.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/146,607, filed Jun. 7, 2005 and titled DEFIBRILLATION PACINGCIRCUITRY, which is a continuation of U.S. patent application Ser. No.10/011,955, filed Nov. 5, 2001, now U.S. Pat. No. 6,952,608 and titledDEFIBRILLATION PACING CIRCUITRY, the entire disclosures of which areincorporated herein by reference.

The invention of the present application may find application in systemssuch as is disclosed in U.S. Pat. No. 6,721,597, titled SUBCUTANEOUSONLY IMPLANTABLE CARDIOVERTER-DEFIBRILLATOR AND OPTIONAL PACER, and U.S.Pat. No. 6,647,292, titled UNITARY SUBCUTANEOUS ONLY IMPLANTABLECARDIOVERTER-DEFIBRILLATOR AND OPTIONAL PACER, and the disclosures ofboth applications are hereby incorporated by reference.

In addition, the foregoing applications are related to: [0004] U.S.patent application Ser. No. 09/940,283, filed Aug. 27, 2001, now U.S.Pat. No. 7,065,407 and titled DUCKBILL-SHAPED IMPLANTABLECARDIOVERTER-DEFIBRILLATOR CANISTER AND METHOD OF USE; [0005] U.S.patent application Ser. No. 09/940,371, filed Aug. 27, 2001, now U.S.Pat. No. 7,039,465 and titled CERAMICS AND/OR OTHER MATERIAL INSULATEDSHELL FOR ACTIVE AND NON-ACTIVE S-ICD CAN; [0006] U.S. patentapplication Ser. No. 09/940,468, filed Aug. 27, 2001, published as US2002-0035379 A1 and titled SUBCUTANEOUS ELECTRODE FOR TRANSTHORACICCONDUCTION WITH IMPROVED INSTALLATION CHARACTERISTICS; [0007] U.S.patent application Ser. No. 09/941,814, filed Aug. 27, 2001, publishedas US 2002-0035381 A1 and titled SUBCUTANEOUS ELECTRODE WITH IMPROVEDCONTACT SHAPE FOR TRANSTHORACIC CONDUCTION; [0008] U.S. patentapplication Ser. No. 09/940,356, filed Aug. 27, 2001, published as US2002-0035378 A1 and titled SUBCUTANEOUS ELECTRODE FOR TRANSTHORACICCONDUCTION WITH HIGHLY MANEUVERABLE INSERTION TOOL; [0009] U.S. patentapplication Ser. No. 09/940,340, filed Aug. 27, 2001, now U.S. Pat. No.6,937,907 and titled SUBCUTANEOUS ELECTRODE FOR TRANSTHORACIC CONDUCTIONWITH LOW-PROFILE INSTALLATION APPENDAGE AND METHOD OF DOING SAME; [0010]U.S. patent application Ser. No. 09/940,287, filed Aug. 27, 2001,published as US 2002-0035377 A1 and titled SUBCUTANEOUS ELECTRODE FORTRANSTHORACIC CONDUCTION WITH INSERTION TOOL; [0011] U.S. patentapplication Ser. No. 09/940,377, filed Aug. 27, 2001, now U.S. Pat. No.6,866,044 and titled METHOD OF INSERTION AND IMPLANTATION OF IMPLANTABLECARDIOVERTER-DEFIBRILLATOR CANISTERS; [0012] U.S. patent applicationSer. No. 09/940,599, filed Aug. 27, 2001, now U.S. Pat. No. 6,950,705and titled CANISTER DESIGNS FOR IMPLANTABLE CARDIOVERTER-DEFIBRILLATORS;[0013] U.S. patent application Ser. No. 09/940,373, filed Aug. 27, 2001,now U.S. Pat. No. 6,788,974 and titled RADIAN CURVE SHAPED IMPLANTABLECARDIOVERTER-DEFIBRILLATOR CANISTER; [0014] U.S. patent application Ser.No. 09/940,273, filed Aug. 27, 2001, now U.S. Pat. No. 7,069,080 andtitled CARDIOVERTER-DEFIBRILLATOR HAVING A FOCUSED SHOCKING AREA ANDORIENTATION THEREOF; [0015] U.S. patent application Ser. No. 09/940,378,filed Aug. 27, 2001, now U.S. Pat. No. 7,146,212 and titled BIPHASICWAVEFORM FOR ANTI-BRADYCARDIA PACING FOR A SUBCUTANEOUS IMPLANTABLECARDIOVERTER-DEFIBRILLATOR; and [0016] U.S. patent application Ser. No.09/940,266, filed Aug. 27, 2001, now U.S. Pat. No. 6,856,835 and titledBIPHASIC WAVEFORM FOR ANTI-TACHYCARDIA PACING FOR A SUBCUTANEOUSIMPLANTABLE CARDIOVERTER-DEFIBRILLATOR, the entire disclosures of whichare incorporated herein by reference.

FIELD

The present invention relates to apparatus and methods useful inconnection with performing electrical cardioversion/defibrillation andoptional pacing of the heart.

BACKGROUND

Defibrillation/cardioversion is a technique employed to counterarrhythmic heart conditions including some tachycardias in the atriaand/or ventricles. Typically, electrodes are employed to stimulate theheart with electrical impulses or shocks, of a magnitude substantiallygreater than pulses used in cardiac pacing.

Defibrillation/cardioversion systems include body implantable electrodesthat are connected to a hermetically sealed container housing theelectronics, battery supply and capacitors. The entire system isreferred to as implantable cardioverter/defibrillators (ICDs). Theelectrodes used in ICDs can be in the form of patches applied directlyto epicardial tissue, or, more commonly, are on the distal regions ofsmall cylindrical insulated catheters that typically enter thesubclavian venous system, pass through the superior vena cava and, intoone or more endocardial areas of the heart. Such electrode systems arecalled intravascular or transvenous electrodes. U.S. Pat. Nos.4,603,705; 4,693,253; 4,944,300; and 5,105,810, the disclosures of whichare all incorporated herein by reference, disclose intravascular ortransvenous electrodes, employed either alone, in combination with otherintravascular or transvenous electrodes, or in combination with anepicardial patch or subcutaneous electrodes. Compliant epicardialdefibrillator electrodes are disclosed in U.S. Pat. Nos. 4,567,900 and5,618,287, the disclosures of which are incorporated herein byreference. A sensing epicardial electrode configuration is disclosed inU.S. Pat. No. 5,476,503, the disclosure of which is incorporated hereinby reference.

In addition to epicardial and transvenous electrodes, subcutaneouselectrode systems have also been developed. For example, U.S. Pat. Nos.5,342,407 and 5,603,732, the disclosures of which are incorporatedherein by reference, teach the use of a pulse monitor/generatorsurgically implanted into the abdomen and subcutaneous electrodesimplanted in the thorax. This system is far more complicated to use thancurrent ICD systems using transvenous lead systems together with anactive can electrode and therefore it has no practical use. It has infact never been used because of the surgical difficulty of applying sucha device (3 incisions), the impractical abdominal location of thegenerator and the electrically poor sensing and defibrillation aspectsof such a system.

Recent efforts to improve the efficiency of ICDs have led manufacturersto produce ICDs which are small enough to be implanted in the pectoralregion. In addition, advances in circuit design have enabled the housingof the ICD to form a subcutaneous electrode. Some examples of ICDs inwhich the housing of the ICD serves as an optional additional electrodeare described in U.S. Pat. Nos. 5,133,353; 5,261,400; 5,620,477; and5,658,321, the disclosures of which are incorporated herein byreference.

ICDs are now an established therapy for the management of lifethreatening cardiac rhythm disorders, primarily ventricular fibrillation(V-Fib). ICDs are very effective at treating V-Fib, but are therapiesthat still require significant surgery.

As ICD therapy becomes more prophylactic in nature and used inprogressively less ill individuals, especially children at risk ofcardiac arrest, the requirement of ICD therapy to use intravenouscatheters and transvenous leads is an impediment to very long termmanagement as most individuals will begin to develop complicationsrelated to lead system malfunction sometime in the 5-10 year time frame,often earlier. In addition, chronic transvenous lead systems, theirreimplantation and removals, can damage major cardiovascular venoussystems and the tricuspid valve, as well as result in life threateningperforations of the great vessels and heart. Consequently, use oftransvenous lead systems, despite their many advantages, are not withouttheir chronic patient management limitations in those with lifeexpectancies of >5 years. The problem of lead complications is evengreater in children where body growth can substantially altertransvenous lead function and lead to additional cardiovascular problemsand revisions. Moreover, transvenous ICD systems also increase cost andrequire specialized interventional rooms and equipment as well asspecial skill for insertion. These systems are typically implanted bycardiac electrophysiologists who have had a great deal of extratraining.

In addition to the background related to ICD therapy, the presentinvention requires a brief understanding of a related therapy, theautomatic external defibrillator (AED) AEDs employ the use of cutaneouspatch electrodes, rather than implantable lead systems, to effectdefibrillation under the direction of a bystander user who treats thepatient suffering from V-Fib with a portable device containing thenecessary electronics and power supply that allows defibrillation. AEDscan be nearly as effective as an ICD for defibrillation if applied tothe victim of ventricular fibrillation promptly, i.e., within 2 to 3minutes of the onset of the ventricular fibrillation.

AED therapy has great appeal as a tool for diminishing the risk of deathin public venues such as in air flight. However, an AED must be used byanother individual, not the person suffering from the potential fatalrhythm. It is more of a public health tool than a patient-specific toollike an ICD. Because >75% of cardiac arrests occur in the home, and overhalf occur in the bedroom, patients at risk of cardiac arrest are oftenalone or asleep and can not be helped in time with an AED. Moreover, itssuccess depends to a reasonable degree on an acceptable level of skilland calm by the bystander user.

What is needed therefore, especially for children and for prophylacticlong term use for those at risk of cardiac arrest, is a combination ofthe two forms of therapy which would provide prompt and near-certaindefibrillation, like an ICD, but without the long-term adverse sequelaeof a transvenous lead system while simultaneously using most of thesimpler and lower cost technology of an AED. What is also needed is acardioverter/defibrillator that is of simple design and can becomfortably implanted in a patient for many years.

Moreover, it has appeared advantageous to the inventor to provide thecapability in such improved circuitry to provide a signal suitable forpacing when the circuitry is not operating in a defibrillation mode.

SUMMARY

Accordingly, the invention relates in various aspects to methods andapparatus for selectively converting a defibrillator circuit or circuitfor delivering a defibrillating pulse to a patient into circuitrysuitable for providing a constant current, useful, e.g., in pacingapplications.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, reference is now made tothe drawings where like numerals represent similar objects throughoutthe figures and wherein:

FIG. 1 is a schematic view of a conventional defibrillator circuit;

FIG. 2 is a circuit schematic of an illustrative embodiment of thepresent invention;

FIG. 3 is a circuit schematic of an alternate embodiment;

FIG. 4 is a circuit schematic of a second alternate embodiment; and

FIGS. 5 and 6 schematically illustrate high side current controllingcircuits.

FIG. 7 is an illustrative example of an implantable cardiac device.

DETAILED DESCRIPTION

FIG. 1 illustrates a conventional “H-bridge” defibrillator circuit 11.The circuit 11 includes a capacitor C.sub.1 which is charged to a highvoltage V.sub.HV and four switches H.sub.1, H.sub.2, L.sub.1, L.sub.2.The capacitor C.sub.1 and switches H.sub.1, H.sub.2, L.sub.1, L.sub.2are used to create either a monophasic voltage pulse or a biphasicvoltage pulse (FIG. 2) across a patient represented by resistanceR.sub.PATIENT. In various applications, the switches H.sub.1, H.sub.2,L.sub.1, L.sub.2, may be MOSFETs, IGBTs, or SCRs (silicon controlledrectifiers).

To create a biphasic waveform such as that shown in FIG. 2, a first pairof switches, e.g., H.sub.1 and L.sub.2, may be closed to create apositive pulse 13. Then all of the switches, H.sub.1, H.sub.2, L.sub.1,L.sub.2, are turned off during a “center pulse” delay period d.sub.1. Atthe end of the delay period d.sub.1, the switches H.sub.2 and L.sub.1are both closed, thereby reversing the current through the patientR.sub.PATIENT to produce a negative voltage pulse 17. Typically, digitallogic is employed to control the sequencing of the switches H.sub.1,H.sub.2, L.sub.1, L.sub.2. In such cases, the order of the pulses can beinverted, i.e., the negative pulse 17 can be produced before thepositive pulse 13. In illustrative applications, the duration of thepulses 13, 17 is, e.g., 1 to 20 milliseconds and the inter-pulse delayd.sub.1 is, e.g., one millisecond.

FIG. 3 illustrates circuitry which may operate as a defibrillatorcircuit during a first selected interval and as a constant currentsource during a second selected interval. The constant current may beuseful, for example, in providing a “pacing” current to a patientR.sub.PATIENT.

As in FIG. 1, the high side switches H.sub.1, H.sub.2 employed in FIG. 3may be IGBTs, MOSFETs, SCRs, or other suitable switches. Such high sideswitches H.sub.1, H.sub.2 may be controlled in any suitable manner suchas, for example, with pulse transformers, opto-couplers, photo-voltaicgenerators, or in accordance with the teachings of U.S. patentapplication Ser. No. 10/011,957, filed on Nov. 5, 2001 on behalf of thesame inventor, now U.S. Pat. No. 6,954,670 and titled SIMPLIFIEDDEFIBRILLATOR OUTPUT CIRCUIT, herein incorporated by reference. Digitallogic suitable for controlling such circuitry to achieve switching maycomprise a programmed microprocessor, microcontroller, discrete logic,or other forms of digital logic control.

In the circuit of FIG. 3, a resistor R.sub.1 is inserted in series withthe emitter or the source leg of a first low side transistor Q.sub.2,which is preferably an IGBT or MOSFET. Similarly, a resistor R.sub.2 isinserted in series with the emitter or source leg of the second low sidetransistor Q.sub.1. A constant voltage is applied across the resistorR.sub.1 via a voltage source, which applies a voltage V.sub.DRIVE to thegate (or base) of the first low side transistor Q.sub.2. Duringoperation of the circuit of FIG. 2 as a defibrillator, the transistorsQ.sub.1, Q.sub.2 serve the purposes of low side switches, e.g., L.sub.1,L.sub.2 of FIG. 1, and the resistors R.sub.1, R.sub.2 are switched outof the circuit by suitable means, e.g., switches SW.sub.1 and SW.sub.2.During pacing operation of the circuit of FIG. 3, a suitable switchingsignal is applied to switch resistor R.sub.1 into the circuit.

In an illustrative application of the circuitry of FIG. 3, the low sidetransistors Q.sub.1, Q.sub.2 may be high voltage IGBTs or MOSFETs,ranging from 500 volts to 3,000 volts capacity or greater. In thecircuit of FIG. 3, the voltage across the resistor R.sub.1 is defined bythe equation:

V.sub.R1=V.sub.DRIVE-V.sub.T  (1)

where V.sub.T is the fixed (constant) threshold voltage of the low sidetransistor Q.sub.1. Thus, if V.sub.DRIVE is 15 volts, and V.sub.T is inthe range of 2-6 volts, V.sub.R1 is in the range of 13 to 9 volts.Accordingly, a constant voltage is applied across the resistor R.sub.1,resulting in a constant current I.sub.RI through the resistor R.sub.1,and hence through the patient R.sub.PATIENT.

As those skilled in the art may appreciate, the threshold voltageV.sub.T of the transistor Q.sub.1 may vary from device to device. Hence,it is typically necessary to calibrate the circuit in production. Incalibrating a circuit like that of FIG. 3, a known voltage is appliedand the current through R.sub.1 is measured, typically resulting in alarge offset, which is compensated for by the system software.

In order to avoid calibration, the voltage source may be constructedusing a feedback circuit employing an operational amplifier as shown inFIG. 4. The op-amp is connected to directly drive the low sidetransistor Q.sub.2, which may comprise, e.g., a MOSFET or IGBT. Use ofthe operational amplifier removes the uncertainty of the thresholdvoltage V.sub.T so that the current that passes through the resistorR.sub.1 is equal to simply V.sub.DRIVE divided by R.sub.1. Thus, one caneither drive the transistor Q.sub.2 with a voltage source and calibratethe system for the V.sub.T of the transistor Q.sub.2 or use an op-ampcircuit to remove the error created by the threshold voltage V.sub.T ofthe transistor Q.sub.2.

During constant current source operation of the circuit of FIG. 4, theappropriate high side switch is on to permit current flow. In addition,the capacitor voltage V.sub.c needs to be appropriately selectedaccording to a number of considerations. First, the current that isprogrammed to go through the patient will generate a voltageV.sub.PATIENT across the patient. Then, in order to make the currentsource work, the voltage compliance V.sub.COMP of the current sourcemust be appropriately set. In the case of FIG. 4, the voltage complianceV.sub.COMP is the voltage V.sub.R1 across the resistor R.sub.1 plus theminimum operating voltage V.sub.T of the low side transistor Q.sub.2.Accordingly, the minimum voltage V.sub.HV across the capacitor C.sub.1is defined by the relation:

V.sub.HV(min.)=V.sub.PATIENT+V.sub.COMP  (2)

The higher V.sub.HV is above V.sub.HV (min.), the closer the currentsource will approach an ideal current source. Another consideration insetting V.sub.HV is power consumption.

The amount of current I.sub.R1 can be varied by varying the voltageV.sub.DRIVE or by switching in different resistors, e.g., in series withor for R.sub.1. From an implementation point of view, it is lessattractive to switch in a resistor because such switching requiresadding transistors or other switching devices. It is more efficient tosimply vary the voltage V.sub.DRIVE. Suitable logic circuitry may beprovided to select the value of V.sub.DRIVE. A DAC (digital to analogconverter) is one example of such logic circuitry. As those skilled inthe art will appreciate, a DAC is a circuit that generates differentvoltages in response to corresponding digital codes provided to it. Sucha DAC could be used to drive either an input of the op-amp A (asillustrated in FIG. 4) or the input (gate) of the transistor Q.sub.1. Asnoted above, an advantage of the op-amp A is that it removes the V.sub.Tterm from the V.sub.HV equation. Particular parameter ranges forcircuitry as configured in FIGS. 3 and 4 include 1 to 50 ohms for theresistance R.sub.1 and 1 to 20 volts for a V.sub.DRIVE resulting in acurrent ranging from 0 to 500 milliamps.

Another illustrative circuit for implementing a current source isillustrated in FIG. 5. This circuit employs a resistor R.sub.3 connectedbetween the high voltage capacitor C.sub.1 and the high side switchesH.sub.1, H.sub.2. The resistor R.sub.3 is switched out of the circuit bya switch SW.sub.3 for defibrillator operation and into the circuit forpacing.

The circuit of FIG. 5 is somewhat more energy wasteful but will workwith the use of a high voltage switch for SW.sub.3. In the circuit ofFIG. 5, the switches H.sub.1, H.sub.2, L.sub.1, L.sub.2 are manipulatedso as to place the resistor R.sub.3 in series with the output. Theamount of current may then be selected by the voltage to which thecapacitor C.sub.1 is charged. As an example, assuming the patientresistance R.sub.PATIENT varies from 30-150 ohms, selecting a resistorR.sub.3 of anywhere from 500-5000 ohms, i.e., a resistance that is muchlarger than that of the patient, results in an approximation of an idealcurrent source. The approximation is:

i=VHVR3+R PATIENT  (3)##EQU00001##

While creation of a current source according to FIG. 5 is relativelyeasy, switching the circuit to the defibrillation mode is more complex.As shown in FIG. 6, a high voltage switch SW.sub.3 is connected acrossthe series resistor R.sub.3 to switch R.sub.3 out of the circuit inorder to enter the defibrillation mode. Since the high voltage switchSW.sub.3 is a floating switch, a high side driver 19 is also needed.These considerations render the circuit of FIG. 6 more difficult toimplement in an implantable device.

In contrast, the circuits of FIGS. 3 and 4 require a switch, e.g.,SW.sub.1 to switch to the defibrillation mode, but the switch SW.sub.1does not have to be a high voltage switch. Instead, the switch SW.sub.1need only be a smaller, low voltage device having the capacity to passthe defibrillation current. In an illustrative circuit, there may be onthe order of only 10 volts across SW.sub.1, which is advantageous.

Thus, only a low voltage switch need be used in the circuits of FIGS. 3and 4. No low voltage driver is necessary since the switch SW.sub.1 isreferenced to ground and can therefore be driven directly. A high sidedriver circuit is unnecessary. In either of the circuits of FIG. 3 orFIG. 4, the voltage V.sub.DRIVE is preferably implemented by a DAC,either connected to directly drive the resistor R.sub.1 (FIG. 3) or todrive the resistor R.sub.3 through an op-amp A (FIG. 4).

Provision of a constant current has the advantage of maintaining aconstant current density across the heart, irrespective of the electrodeinterface impedance.

FIG. 7 illustrates an implantable cardiac device 100. The implantablecardiac device includes a housing 102 for containing the operationalcircuitry of the device. Attached to the housing 102 is a lead 104carrying a plurality of electrodes 106. The implantable cardiac device100 is merely illustrative of one design for an implantable device. Thehousing 102 may, if desired, be an active housing having an electrodefor stimulus delivery or sensing. Other details of the implantablecardiac device 100 are found in U.S. Pat. No. 6,721,597, which isincorporated herein by reference.

While the present invention has been described above in terms ofspecific embodiments, it is to be understood that the invention is notlimited to the disclosed embodiments. On the contrary, the followingclaims are intended to cover various modifications and equivalentmethods and structures included within the spirit and scope of theinvention.

1. (canceled)
 2. An implantable defibrillator comprising: an H-bridgecircuit comprising a high side and a low side, wherein the H-bridgecircuit comprises first and second legs connected between the high sideand the low side thereof, wherein the first leg of the H-bridge circuitcomprises first and second current switching elements and the second legof the H-bridge circuit comprises at least a third current switchingelement, the H-bridge comprising output nodes for coupling to patienttissue via implantable electrodes, a first output node located on thefirst leg between the first and second current switching elements and asecond output node located on the second leg; wherein one of the firstand second current switching elements along with the third currentswitching element define a pair of current switching elements such that,when a constant current pacing stimulus is desired, the defibrillator isconfigured to select the pair of current switching elements to generatea pacing stimulus of a first polarity for application via the first andsecond output nodes; and a constant current source connected to one ofthe high side or the low side of the H-bridge circuit, wherein theconstant current source comprises a voltage source and a controlledcurrent output circuit connected between the voltage source and theH-bridge circuit, and the defibrillator is configured to apply a controlsignal to the constant current source to generate a substantiallyconstant current output via the output nodes.
 3. The implantabledefibrillator of claim 2 wherein the H-bridge circuit further comprisesa fourth current switching element, the fourth current switching elementdisposed in the second leg such that the second output node is betweenthe third and fourth current switching elements.
 4. The implantabledefibrillator of claim 3 wherein the constant current source isconnected to the high side of the H-bridge circuit to control currentflowing into the H-bridge circuit.
 5. The implantable defibrillator ofclaim 4 wherein the constant current source comprises a switch connectedin parallel with a resistor.
 6. The implantable defibrillator of claim 3wherein the constant current source is connected to the low side of theH-bridge circuit to control current flowing through at least one legthereof.
 7. The implantable defibrillator of claim 3 wherein: in thefirst leg of the H-bridge, the first current switching element links thehigh side of the H-bridge to the first output node and the secondcurrent switching element links the low side of the H-bridge to thefirst output node; in the second leg of the H-bridge, the third currentswitching element links the high side of the H-bridge to the secondoutput node and the fourth current switching element links the low sideof the H-bridge to the second output node; the constant current sourceincludes an amplifier for generating a control signal output; and thedefibrillator is configured to selectively provide the control signaloutput to the fourth switching element.
 8. The implantable defibrillatorof claim 7 wherein the defibrillator is configured to generate asubstantially constant current pacing therapy by applying a firstvoltage to the high side of the H-bridge, closing the first switch, andapplying the control signal output to the fourth switch.
 9. Theimplantable defibrillator of claim 8 wherein the defibrillator comprisesa therapy circuit with a high power capacitor for use in deliveringeither pacing or defibrillator therapy, such that the first voltage isreceived from the high power capacitor.
 10. The implantabledefibrillator of claim 3 wherein: in the first leg of the H-bridge, thefirst current switching element links the high side of the H-bridge tothe first output node and the second current switching element links thelow side of the H-bridge to the first output node; in the second leg ofthe H-bridge, the third current switching element links the high side ofthe H-bridge to the second output node and the fourth current switchingelement links the low side of the H-bridge to the second output node;the constant current source includes a voltage source for generating acontrol signal output; and the defibrillator is configured toselectively provide the control signal output to the fourth switchingelement.
 11. The implantable defibrillator of claim 10 wherein thedefibrillator is configured to generate a substantially constant currentpacing therapy by applying a first voltage to the high side of theH-bridge, closing the first switch, and applying the control signaloutput to the fourth switch.
 12. The implantable defibrillator of claim11 wherein the defibrillator comprises a therapy circuit with a highpower capacitor for use in delivering either pacing or defibrillatortherapy, such that the first voltage is received from the high powercapacitor.
 13. The implantable defibrillator of claim 3 wherein: in thefirst leg of the H-bridge, the first current switching element links thehigh side of the H-bridge to the first output node and the secondcurrent switching element links the low side of the H-bridge to thefirst output node, with a first resistor and a first defibrillatorswitch parallel to one another and between the second current switchingelement and the low side of the H-bridge; in the second leg of theH-bridge, the third current switching element links the high side of theH-bridge to the second output node and the fourth current switchingelement links the low side of the H-bridge to the second output nodewith a second resistor and a second defibrillator switch parallel to oneanother and between the second current switching element and the lowside of the H-bridge; and the implantable defibrillator is configured toselect between defibrillation output and controlled current output byopening or closing the first and second defibrillator switches.
 14. Theimplantable defibrillator of claim 13 wherein the defibrillator isconfigured to selectively close the first and second defibrillatorswitches to deliver defibrillation output, and to selectively open thefirst and second defibrillator switches to deliver controlled currentoutput.
 15. The implantable defibrillator of claim 2 wherein the firstleg of the H-bridge is configured with the first switch linking the highside of the H-bridge to the first output node, and the second switchlinking the low side of the H-bridge to the first output node, with aresistor dispose between the second switch and the low side of theH-bridge and placed in parallel with a defibrillator switch, whereindefibrillator switch is controllable by the defibrillator as follows:for delivery of a defibrillation therapy, the implantable defibrillatoris configured to close the defibrillator switch; and for delivery of acontrolled current output, the implantable defibrillator is configuredto open the defibrillator switch to force current through the resistor.16. An implantable defibrillator comprising: a high power capacitorconfigured for delivery of controlled current output and defibrillationtherapy; an H-bridge circuit to selectively couple the high powercapacitor to outputs for delivery of therapy to a patient in at leastcontrolled current output and defibrillation therapy modes, the H-bridgecircuit having a high side and a low side; a selectable control circuitcoupling the high power capacitor to the H-bridge circuit, theselectable control circuit including a resistor for use in providing acontrolled current output and a switch for use in deliveringdefibrillation therapy; wherein the defibrillator is configured tocontrol a voltage to which the high power capacitor is charged forproviding the controlled current output.
 17. The defibrillator of claim16 wherein the defibrillator is configured to close the switch in theselectable control circuit in order to deliver a defibrillation therapy.18. The defibrillator of claim 16 wherein the H-bridge circuit comprisesa first leg having first and second switches and a second leg havingthird and fourth switches, with output nodes for electrically couplingto implantable electrodes disposed on the first leg between the firstand second switches and on the second leg between the third and fourthswitches, the output nodes for delivering electrical therapy to thepatient.
 19. An implantable defibrillator comprising: an H-bridgecircuit having: a first leg having a first switch and a second switchand defining a first output node between the first switch and the secondswitch; a second leg having a third switch and a fourth switch anddefining a second output node between the third switch and the fourthswitch; a capacitor for delivering pacing and/or defibrillation therapyto a patient using at least the H-bridge; means for current controlcoupled to the H-bridge circuit, the means for current control having atleast two configurations in which: in the first configuration, the meansfor current control controls a quantity of current through the H-bridgecircuit for delivery to a patient via the output nodes to deliver asubstantially constant current; and in the second configuration, themeans for current control does not control current through the H-bridgesuch that a defibrillation therapy may be delivered.
 20. The implantabledefibrillator of claim 19 wherein the means for current control isdisposed to couple the capacitor for delivering pacing and/ordefibrillation therapy to the H-bridge.
 21. The implantabledefibrillator of claim 19 wherein: the H-bridge has a high side coupledto the capacitor for delivering pacing and/or defibrillation therapy tothe patient and a low side; and the means for current control is coupledto the H-bridge on a low side thereof.